Handover in a wireless local area network (WLAN)

ABSTRACT

In triggering a handoff by a wireless transmit/receive unit (WTRU) from a current basic service set (BSS) in a wireless local area network (WLAN), the following are performed. A highest class of traffic service and quality of service (QoS) is determined for the highest class from a basic service set (BSS) beacon. Handoff is terminated and communication is retained with a current BSS when the signal to noise ratio (SNR) or received signal strength (RSS) is greater than a high threshold of the highest class. Other criteria is evaluated to determine whether a handoff is desired when the SNR or RSS is less than the high threshold.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. provisional application No.60/531,513, filed Dec. 19, 2003, which is incorporated by reference asif fully set forth.

FIELD OF INVENTION

The present invention relates wireless communication systems. Inparticular, the invention relates to handover in such systems.

BACKGROUND

FIG. 1 is a simplified illustration of a wireless transmit/receive unit(WTRU) 14 _(1N) potentially handing over between two basic service sets(BSSs), BSS₁ 12 ₁ and BSS₂ 12 ₂, in a wireless local area network(WLAN). Originally, BSS₁ 12 ₁ has an access point (AP) 10 ₁ and aplurality of WTRUs 14 ₁₁ to 14 _(1N) and BSS₂ 12 ₂ has an access point(AP) 10 ₂ and a plurality of WTRUs 14 ₂₁ to 14 ₂₃. The WTRU 14 _(1N) isin wireless communication with AP 10 ₁. As illustrated in FIG. 1, bothAPs 10 ₂, 10 ₁ are connected to a distribution system 16. To decidewhether to handover between BSSs 12, such as BSS₁ 12 ₁ and BSS₂ 12 ₂,the WTRU 14 _(1N) measures the received signal strength (RSS) or signalto noise ratio (SNR) for each BSS 12 ₁, 12 ₂. The BSS 12 having thebetter RSS or SNR is selected for further communication. If BSS₁ 12 ₁ isselected, the current communication links are maintained, as illustratedas a solid line. If BSS₂ 12 ₂ is selected, a new link is establishedwith BSS₂, as illustrated as a dashed line.

Although this approach most likely provides the WTRU 14 _(1N) with thestrongest link, other criteria may make such a connection undesirable.To illustrate, the BSS having the strongest link may be overloaded andcan not meet some quality of service (QoS) requirements of the WTRU 14_(1N). Accordingly, it is desirable to have alternate handover schemes.

SUMMARY

In triggering a handoff by a wireless transmit/receive unit (WTRU) froma current basic service set (BSS) in a wireless local area network(WLAN), the following are performed. A highest class of traffic serviceand quality of service (QoS) is determined for the highest class from abasic service set (BSS) beacon. Handoff is terminated and communicationis retained with a current BSS when the signal to noise ratio (SNR) orreceived signal strength (RSS) is greater than a high threshold of thehighest class. Other criteria is evaluated to determine whether ahandoff is desired when the SNR or RSS is less than the high threshold.

BRIEF DESCRIPTION OF THE DRAWING(S)

The present invention will be understood from consideration of theaccompanying figures, wherein like elements are designated by likenumerals, and wherein:

FIG. 1 is an illustration of a WTRU in potential handover.

FIG. 2 is a flow chart of an embodiment of a RSS/SNR and other systemstatistic handover algorithm.

FIG. 3 is a simplified diagram of an embodiment of a WTRU capable ofRSS/SNR and other system statistic handover.

FIG. 4 is a flow chart of a RSS/SNR and other system statistic handoveralgorithm embodiment.

FIG. 5 is a flow chart of an embodiment of an algorithm for calculationof a QoS index, which may be employed by FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, station, mobile station, fixed or mobilesubscriber unit, pager, or any other type of device capable of operatingin a wireless environment. When referred to hereafter, an access pointincludes but is not limited to a base station, Node-B, site controller,or any other type of interfacing device in a wireless environment.Although the following is discussed with respect to WLANs, the inventioncan be applied to other wireless networks.

FIG. 2 is an embodiment of a RSS/SNR and other system statistichandover. A WTRU, such as WTRU 14 _(1N) of FIG. 1, initiates thehandover algorithm to determine whether handing over between BSSs 12 isdesirable, such as from BSS₁ 12 ₁ to BSS₂ 12 ₂, step S30. The RSS and/orSNR is measured for each BSS 12, including the current BSS and anypotential handover BSSs, step S32. Other system statistics are measuredfor each BSS 12, step S34. The other system statistics may relate to thequality of service, such as delay bounds, bandwidth requirements (i.e.data rate), and frame error rate. Based on the RSS/SNR and other systemstatistics, a handover decision is made, step S36. Typically, the othersystem statistics are based on the traffic class of the WTRU's services.

FIG. 3 is an embodiment of a WTRU 18 capable of such a handover. Thecomponents of FIG. 3 may be implemented on a single integrated circuit(IC), such as an application specific integrated circuit (ASIC), onmultiple ICs, by discrete components or a combination of IC(s) anddiscrete component(s). Wireless signals are received and transmittedover an antenna 20 or antenna array and a transceiver (Xceiver) 22 ofthe WTRU 18. A RSS/SNR measuring device 24 measures the RSS and/or SNRof each BSS 12. A handover controller 26 receives the RSS/SNRmeasurements and other system statistics and determines whether ahandover to another BSS 12 is desired. The other system statistics maybe recovered from received communications, as shown in FIG. 3 or byother means.

FIG. 4 is an illustration of a preferred embodiment for RSS/SNR andother system statistic handover. For each traffic channel, QoScharacters are defined, such as delay bounds, bandwidth requirements(data rate), and frame error rate. Minimum and maximum values for eachparameter are defined for each traffic class. A minimum and maximumvalue of SNR is also defined for each traffic class. Table 1 illustratesan example of QoS characteristics and SNR values for different trafficclasses.

TABLE 1 QoS Characteristics and SNR definition for different trafficclasses Frame Error Rate Delay (D) Data Rate (BW) (FER) SNR TrafficClass_1 Dmin Dmax BWmin BWmax FERmin FERmax SNRminSNRmax - - - - - - - - - - - - - - - - - - - - - - - - Traffic Class_nDmin Dmax BWmin BWmax FERmin FERmax SNRmin SNRmax

The handover algorithm is triggered when the SNR value drops below ahigh SNR threshold, i.e., SNR max, for the given traffic class (TC)associated with the entity seeking a handover, step S40. The TC may beone of those shown in Table 1. The algorithm compares the SNR value witha low SNR threshold and depending on the result acts, generally, asfollows.

If the SNR value is between the low and high SNR thresholds, thealgorithm checks the QoS index for this traffic class. The QoS index maybe derived from any or all the criteria in Table 1 or, alternately,other criteria may be used. If the QoS index is below the QoS indexthreshold, the WTRU starts scanning neighboring cells to trigger ahandover. If the SNR value is higher than the high SNR threshold, thealgorithm terminates since link quality is good and there is no need forhandover. For SNR values below the low threshold, the WTRU startsscanning neighboring BSSs without comparing the QoS index with the QoSindex threshold. Although the above refers to SNR, RSS or a combinationof RSS and SNR may be used instead.

Referring to FIG. 4, the highest class of service for traffic at themonitoring WTRU 18 and the QoS requirements of the WTRU 18 are examined,step S42. If the SNR is at or above the low threshold, step S44, thechannel utilization and the frame loss rate from the QBSS load elementis determined, step S48. The QoS parameter set element is checked, stepS50, and the QoS index is calculated, step S52. If the QoS index isgreater than a QoS index threshold, the handover algorithm is ended,steps S54, S86. If the QoS index is less than or equal to the threshold,the algorithm proceeds to determining a list of neighboring BSSs 12 toscan as described subsequently, for steps S62 to S84.

If the SNR is below the low threshold, step S44, the channel utilizationis determined and frame loss rate derived from the QBSS load element,step S56. The QoS parameter set element is checked, step S58, and theQoS index is calculated, step 60.

A list of neighbor BSSs 12 is determined, step S62, and a scan neighborroutine is initiated, step S64. The first BSS 12 of the list is scanned,step S66. The probe response is obtained from the first BSS 12 and theframe loss rate, channel utilization and QoS parameters are obtainedfrom the probe response, step S68. The SNR and QoS parameter elementsare checked, step S70. A QoS index calculation for the first BSS 12 ofthe neighbors to be scanned is performed, step S72.

In the event that there are more BSSs 12 in the list, step S74, the nextBSS 12 is picked, step S76. Steps S68 through S74 are repeated for thenext BSS 12.

When there are no more BSSs 12 to be scanned, the BSS 12 with thehighest QoS index is picked, at step S78. A difference is taken betweenthe QoS index of the selected BSS 12 and the QoS index of the currentBSS 12. To keep the WTRU 18 from frequently handing over between BSSs12, the QoS index difference value is compared with a hysteresis todetermine if it is bigger than the hysteresis, step S80. The hysteresisis preferably a function of the traffic class (TC), although it may bederived by other techniques. If the calculated difference is greaterthan the last stored hysteresis, the handover to the new cell isinitiated and the hysteresis value is reset to its original value, stepS82. The handoff algorithm terminates, step S86. If the differencebetween the current and target cell QoS indexes is smaller than thehysteresis, the hysteresis value is updated, step S84. Preferably, thehysteresis value is decreased in order to enable the WTRU 18 utilizingthe handover algorithm to have a better chance to obtain a handover to anew cell in the event that the WTRU 18 continues to experience poorservice.

An embodiment of a QoS index calculation algorithm is shown in FIG. 5.Although the algorithm can be used in other applications, it ispreferably used with steps S52 and S72 of FIG. 4. The QoS index isinitially set to zero, steps S88, S90, and a list of available QoSparameters is created, step S92. The first QoS parameter in the list isselected, step S94. The selected QoS parameter is compared with the highthreshold taken from the associated traffic class (TC), step S96. If theselected parameter is greater than the high threshold, the QoS index isincremented, step S98. Alternatively, if the QoS parameter is less thanthe high threshold and less than the low threshold, step S100, the QoSindex remains unchanged. If the QoS parameter is less than both the highand low threshold, the present QoS index is decreased by n+1, where n isthe total number of BSSs being examined, step S102. After one of thesethree (3) steps, S90, S100, S102 has been performed, it is determined ifthere are any more QoS parameters to be examined, step S104. In theevent that there are more QoS parameters, the next QoS parameter isselected, step S106. Steps S96 to S104 are repeated until all of the QoSparameters have been examined. After all of the QoS parameters have beenevaluated, the QoS index is produced, step S108.

Although FIG. 5 is one embodiment for producing a QoS index, others maybe used. For example, the QoS index may be produced by weighting QoSparameters.

One application of the algorithms in FIGS. 4 and 5 can be with an802.11e compliant AP and WTRU. Additionally, another application is withan 802.11b AP and WTRU with the needed parameters for the algorithmadded to the 802.11 beacon and probe response frames or throughproprietary signaling. These algorithms can be also applied to otherwireless environments.

1. A method for determining handoff by a wireless transmit/receive unit(WTRU) from a current basic service set (BSS) in a wireless local areanetwork (WLAN), comprising: determining a highest class of trafficservice and quality of service (QoS) for the highest class from a basicservice set (BSS) beacon; obtaining channel utilization and frame lossrate from a QBSS load element provided in the beacon; obtaining the QoSparameter set and average delay in a current cell contained in thebeacon; calculating a QoS index employing data including one or more of:channel utilization, frame loss and delay of the current BSS, andwherein the calculating a QoS index further comprises a) setting a QoSindex to zero (0); b) creating a list of QoS parameters; c) selectingone of the QoS parameters; d) comparing the selected QoS parameter witha high threshold for that QoS parameter; and e) performing one ofincrementing the QoS index when the selected QoS parameter is greaterthan the high threshold for that QoS parameter; retaining a current QoSindex when the selected QoS parameter is equal to the threshold for thatQoS parameter; and reducing the current QoS index; and terminating ahandoff and retaining communication with the current BSS when thecalculated QoS index is at least equal to a QoS index threshold.
 2. Themethod of claim 1 wherein the current QoS index is reduced by (n+1)where n is equal to a number of available QoS parameters.
 3. The methodof claim 1 further comprising: terminating a handoff and maintainingcommunication with the current BSS when the QoS index is at least equalto the QoS index of the current BSS.
 4. The method of claim 1 furthercomprising: continuing a handoff operation when the QoS index is lessthan the QoS index of the current BSS.
 5. The method of claim 4 whereinthe continuing the handoff operation comprises: obtaining a list ofneighboring BSSs; for each BSS from the list; obtaining information fromthe selected BSS needed for calculating the QoS index; and initiatinghandoff when the highest QoS index is greater than a hysteresis QoSthreshold value.
 6. The method of claim 5 further comprising: loweringthe hysteresis QoS threshold to facilitate subsequent handoffprocedures.
 7. A wireless transmit/receive unit (WTRU) comprising: meansfor determining a highest class of traffic service and quality ofservice (QoS) for the highest class from a basic service set (BSS)beacon; means for obtaining channel utilization and frame loss rate froma QBSS load element provided in the beacon; means for obtaining the QoSparameter set and average delay in a current cell contained in thebeacon; means for calculating a QoS index employing data including oneor more of: channel utilization, frame loss and delay of the currentBSS, and wherein the calculation of the QoS index further comprises: a)setting a QoS index to zero (0); b) creating a list of QoS parameters;c) selecting one of the QoS parameters; d) comparing the selected QoSparameter with a high threshold for that QoS parameter; and e)performing one of incrementing the QoS index when the selected QoSparameter is greater than the high threshold for that QoS parameter;retaining a current QoS index when the selected QoS parameter is equalto the threshold for that QoS parameter; and reducing the current QoSindex; and means for terminating a handoff and retaining communicationwith the current BSS when the calculated QoS index is at least equal toa QoS index threshold.
 8. The WTRU of claim 7 wherein the current QoSindex is reduced by (n+1) where n is equal to a number of available QoSparameters.
 9. The WTRU of claim 7 further comprising: means forterminating a handoff and maintaining communication with the current BSSwhen the QoS index is at least equal to the QoS index of the currentBSS.
 10. The WTRU of claim 7 further comprising: means for continuing ahandoff operation when the QoS index is less than the QoS index of thecurrent BSS.
 11. The WTRU of claim 10 wherein the continuing the handoffoperation comprises: obtaining a list of neighboring BSSs; for each BSSfrom the list; obtaining information in a mode response from theselected BSS needed for calculating the QoS index; and initiatinghandoff when the highest QoS index is greater than a hysteresis QoSthreshold value.
 12. The WTRU of claim 11 further comprising: means forlowering the hysteresis QoS threshold to facilitate subsequent handoffprocedures.